Ah Ha, OK so I'm running couriertcpd
p4-7126:~# netstat -anp | grep 143
tcp 0 0 0.0.0.0:143 0.0.0.0:* LISTEN
26673/couriertcpd
unix 2 [ ] DGRAM 2864143 21874/imapd
Now all I need to know is Why? And how do I stop it so uw-imap will
run on port 143 instead?

I think you should `apt-get install courier-imap` (or other courier
packages if necesary), and after that do a `dpkg -P courier-imap`. It
will wipe out the courier imapd, and you can use the desired imapd.